I use a 1080p monitor and what I’ve noticed is that once creators start uploading 4k content the 1080p version that I watch on fullscreen has more artifacting than when they only uploaded in 1080p.
Did you notice that as well?
Watching in 1440p on a 1080p monitor results in a much better image, to the detriment of theoretically less sharper image and a lot higher CPU usage.
AVI is a container, not a codec. An AVI container can contain video encoded with any kind of codec (unlike some other container formats, which are more restrictive). If you want to, you could put e.g. a VP9 or AV1 video stream (so the ones that YouTube is using) into an AVI container. In theory at least, if you uploaded an AVI file containing VP9 video, YouTube could just extract it from the container and stream it as is, but they’ll still re-encode it. Before you think that all of this talk of modern codecs in AVI containers is theoretical, AVI is used a a standard for archiving with some institutions, so it’s more relevant than you might think.
However, you are partially right in that AVI can not be used for streaming, not just by YouTube, but in general, since this requirement obviously wasn’t taken into account when it was introduced in 1992 and thus not incorporated into this standard.